MEMS (Micro-Electro-Mechanical Systems) accelerometers, also sometimes referred to as Highly Integrated Accelerometers (HIA), are used in a variety of applications including as triggering sensors for air bag deployment. An HIA is designed to sense changes in acceleration at a defined sensitivity threshold. Events that satisfy the defined acceleration criteria electronically activate a signal, which in turn is used to initiate a desired device response, such as, for example, inflation of a safety air bag.
A MEMS accelerometer typically includes structures known as sensing fingers. In a common design, the sensing fingers are fabricated of silicon from the underlying silicon substrate of the electronic device. A sensing finger is a three dimensional structure, typically with high aspect ratio, and in the general shape of a wall. A sensing finger is designed to deflect physically in response to a sensed acceleration. Thus, it is typically desired to fabricate a sensing finger, or preferably an array of sensing fingers, with a given spacing and resistance to deflection.
Typically, an array of sensing fingers is integrally created in an HIA. Electrical charge in the finger array creates a capacitance between adjacent fingers. Rapid accelerations of the HIA result in a physical deflection of neighboring fingers. This physical deflection affects the capacitance of the array. The device to which the array is attached senses the change in capacitance, and this initiates the device signal.
One drawback to current methods of fabricating sensing fingers is finger stiction. If two or more fingers bend or deflect, they may come into contact with one another. The contact can also result in the fingers adhering to each other. This is finger stiction. Stiction can arise for a variety of reasons including capillary forces, electrostatic forces, and Van der Walls attraction. Stiction is undesirable for the reason that it leads to failure of the accelerometer. Contact between fingers can cause an electrical short thereby upsetting the designed electrical function of the device. Additionally, the result of two fingers in contact results in a mechanical stiffening of the structure, which may itself affect the designed deflection resistance of the device.
There is a further movement in the design of HIAs and MEM accelerometers to increase the aspect ratio of the silicon fingers such that the fingers grow in height for a given width. This trend results from a desire to decrease the footprint of the finger array on a base without decreasing the capacitance area in the array. One way to achieve this is to increase the vertical height of a silicon finger. However, finger elongation further aggravates stiction problems; it creates a physical setting in which any bending of the silicon fingers are additionally susceptible to stiction. The geometry of elongated fingers lowers the threshold at which bending or warping places adjacent fingers in contact.
It is further desired to improve the sensitivity of HIAs. As these devices are often used in the triggering of safety equipment, it is desired to improve the functional sensitivity if possible. Generally sensitivity of a finger array is a function of displacement/acceleration. Additionally the sensitivity may be characterized as mass of the finger/finger spring constant. Thus, adding mass to silicon fingers has the added benefit of improving the device sensitivity.
Accordingly, it is desirable to redesign currently used accelerometers. In particular it is desired to design and manufacture an accelerometer so as to reduce finger stiction. In addition, it is desirable to design an accelerometer with improved sensitivity. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.